[PATCH RESEND] writeback: account BDI's min_ratio for unregistered device

[Akinobu Mita <akinobu.mita@gmail.com>]

>From mm/page-writeback.c:

	/*
	 * bdi_min_ratio keeps the sum of the minimum dirty shares of all
	 * registered backing devices, which, for obvious reasons, can not
	 * exceed 100%.
	 */
	static unsigned int bdi_min_ratio;

But when a backing device is unregistered, the BDI's min_ratio is not
subtracted from bdi_min_ratio.  So the second attempt to set min_ratio
in the following commands will fail.

	# modprobe null_blk
	# echo 50 > /sys/block/nullb0/bdi/min_ratio
	# modprobe -r null_blk
	# modprobe null_blk
	# echo 50 > /sys/block/nullb0/bdi/min_ratio

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
---
Resending with Tejun's ACK

 mm/backing-dev.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 885361b..ccf72be 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -849,6 +849,7 @@ void bdi_destroy(struct backing_dev_info *bdi)
 	}
 
 	wb_exit(&bdi->wb);
+	bdi_set_min_ratio(bdi, 0);
 }
 EXPORT_SYMBOL(bdi_destroy);
 
-- 
1.9.1